Methods for blow molding solid-state cellular thermoplastic articles

ABSTRACT

A process for producing cellular thermoplastic articles. The process comprises the steps of treating a solid parison made from a thermoplastic material with a saturating gas at an elevated pressure for a period of time to provide a gas-saturated parison; heating the gas-saturated parison to prepare a cellular parison; placing the cellular parison in a mold; and blowing a molding gas into the cellular parison to expand the cellular parison into the shape of the mold to provide a shaped cellular article.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application No.60/971,844, filed Sep. 12, 2007, which is incorporated herein byreference in its entirety.

BACKGROUND

Blow molding is a manufacturing process used to produce hollow articlesfrom thermoplastics. Various methods of blow molding hollow articles areknown. Typically, the blow molding process begins with melting athermoplastic material and forming a parison of the thermoplastic. Theparison is then clamped into a mold and air or a gaseous medium ispumped into the parison by the use of nozzles that have been insertedinto the parison. The gaseous pressure pushes the thermoplastic materialoutward to match the shape of the mold. Once the thermoplastic materialhas cooled and hardened, the mold is opened up and the article isejected.

Various methods are known for producing cellular thermoplastic articleswhich are generally referred to as “foam” or “foamed thermoplastic”articles. Methods for producing cellular thermoplastic articles include,for example, the use of foaming agents that release gases that expandthe thermoplastic materials at their normal processing temperatures; theuse of thermoplastic materials containing liquids or solids, or both,that can be removed by extraction or dissolution; and the technique ofstretching thermoplastic films containing liquids or solids, or both, toproduce interfacial voids, followed by extraction or dissolution.However, the existing processes of making foamed hollow articles sufferfrom one common problem, which is that the foamed parisons do not havesufficient wall strength. In addition, the methods that depend onextraction or dissolution require the formation of an interconnectingnetwork of pores that allows removal of the dissolved liquids or solids.

When molding cellular thermoplastic articles, it is important to produceuniform cell sizes. Both durability and strength of the molded articlesare dependent upon cell size and uniformity. In the blow molding ofcellular articles, the usual manner of controlling cell size anduniformity include altering the foaming agent, pressure and temperatureof extrusion, and changes to the mixing portion of the extruder. Inspite of the various efforts, a need still exists for a process toproduce cellular thermoplastic articles containing pores or cells ofuniform distribution.

The object of the present invention is to meet the above defined needsand provide further related advantages.

SUMMARY OF THE INVENTION

The present invention provides a process for producing a cellulararticle from a solid thermoplastic material.

In one embodiment, the process includes the steps of:

(a) treating a solid parison made from a thermoplastic material with asaturating gas at an elevated pressure for a period of time to provide agas-saturated parison;

(b) heating the gas-saturated parison to prepare a cellular parison;

(c) placing the cellular parison in a mold; and

(d) blowing a molding gas into the cellular parison to expand thecellular parison into the shape of the mold to provide a shaped cellulararticle.

The thermoplastic material includes, but is not limited to,polycarbonate, polypropylene, polyethylene, polyethylene terephthalate,polyvinyl chloride, poly(lactic acid), acrylonitrile butadiene styrene,or polystyrene.

The saturating gas may include carbon dioxide, nitrogen, or anycombination thereof. In one embodiment, the saturating gas consistsessentially of carbon dioxide. In another embodiment, the saturating gasconsists essentially of nitrogen.

The solid parison may be treated with the saturating gas at the elevatedpressure ranging from about 3 MPa to about 7.5 MPa. In one embodiment,the elevated pressure is about 4 MPa. In another embodiment, theelevated pressure is about 5 MPa. The treatment of a solid parison witha saturating gas can be carried out in a pressure vessel filled with thesaturating gas. The treatment can proceed to complete saturationfollowed by a step for desorption, or alternatively, the treatment canproceed to partial saturation followed by a step for desorption.

A plurality of solid parisons may be saturated with the saturating gassimultaneously at an elevated pressure to provide a plurality ofgas-saturated parisons. In one embodiment, multiple solid parisons arenested one inside another. In another embodiment, multiple solidparisons are partially nested one inside another. In yet anotherembodiment, each parison has a shape that is smaller at the bottom thanat the top to allow nesting of one parison inside another. When multiplesolid parisons are nested one inside another, the parisons can includemeans for enabling the saturating gas to contact the entire surfaces ofall parisons. Such means can include tabs or ribs on the parisons toprevent contact.

The solid parison may include a body portion and a neck portion. In oneembodiment, the neck portion further comprises screw threads.

The gas-treated parison may be heated at a temperature of about 110° C.In one embodiment, only the body portion of the gas-saturated parison isheated to provide a cellular parison having a cellular body portion anda solid neck portion.

In one embodiment, after treating with the saturating gas to achievecomplete saturation and before heating, the parison is allowed topartially desorb some of the saturating gas.

A variety of inert gases may be useful as the molding gas. In oneembodiment, the molding gas is selected from the group consisting ofnitrogen, argon, xenon, krypton, helium, and carbon dioxide. In anotherembodiment, the molding gas is compressed air.

The process provides a cellular thermoplastic article comprising cellshaving varied sizes. In one embodiment, the cells have a size rangingfrom about 5 μm to about 200 μm. In another embodiment, the cells have asize ranging from about 50 μm to about 150 μm. In yet a furtherembodiment, the cells have a size ranging from about 50 μm to about 100μm.

In one embodiment, the process may further comprise the step ofinjecting a molten thermoplastic material into a heated perform moldaround a hollow mandrel blow tube to provide a solid parison before step(a).

In one embodiment, step (d) of the process may further comprisemechanically stretching the cellular parison with a plunger whileblowing a molding gas into the cellular parison.

The cellular articles and processes to make them disclosed herein may beused as products, such as bottles, jars, and other containers, forexample.

This summary is provided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription. This summary is not intended to identify key features ofthe claimed subject matter, nor is it intended to be used as an aid indetermining the scope of the claimed subject matter.

DESCRIPTION OF THE DRAWINGS

The foregoing aspects and many of the attendant advantages of thisinvention will become more readily appreciated as the same become betterunderstood by reference to the following detailed description, whentaken in conjunction with the accompanying drawings, wherein:

FIG. 1 shows a flow diagram of a traditional injection blow moldingprocess and a stretch blow molding process;

FIG. 2 shows a flow diagram of one embodiment of a blow molding process;

FIG. 3 shows a diagrammatical illustration of a system for producing ablow molded cellular thermoplastic article from a solid thermoplasticparison;

FIG. 4 is a diagrammatical illustration of a plurality of nested solidparisons;

FIG. 5 is a diagrammatical illustration of a cross section of a wall ofa solid cellular parison; and

FIG. 6 is a diagrammatical illustration of a cross section of a wall ofa solid blow molded cellular thermoplastic.

DETAILED DESCRIPTION OF THE INVENTION

The disclosure relates to a process for producing a blow molded cellulararticle from a solid thermoplastic material. A process is disclosed thatimproves upon the traditional blow molding and stretch blow moldingprocesses to provide a blow molded shaped cellular article from a solidthermoplastic material.

The traditional process of blow molding, as diagramed in FIG. 1, can beused for the production of hollow solid thermoplastic articles such asbottles and jars. First, molten thermoplastic material 100 is injectedinto a heated preform mold 105 around a hollow mandrel blow tube toprovide a preformed parison 110. The preform mold forms the externalshape of the parison. The parison is clamped around the mandrel, whichforms the internal shape of the parison. The parison usually includes afully formed bottle/jar neck with a thick tube of thermoplastic materialattached, which will form the body. Then, the parison is placed in alarger blow mold for blow molding 115. The apparatus for the blowmolding step may be a preform-mandrel assembly, in which the parison isexpanded with, for example, compressed air to achieve the finishedarticle shape. Finally, after a cooling period 120, the blow mold opensand the finished shaped solid noncellular thermoplastic article 125 isremoved from the assembly. Depending on the material, the parison mayundergo a cooling step between the parison production and blow moldingsteps. This is because the material may not have the strength to godirectly from a molten state to a blow molding process. Such parison isallowed to cool and then re-heated and blow molded.

Stretch blow molding is a type of blow molding, in which a parison iselongated mechanically in the mold and then expanded radially in ablowing process. Still referring to FIG. 1, in the stretch blow moldingprocess, the molten thermoplastic material 100 flows into the heatedpreform mold 105 via a hot runner block, to produce the desired shape ofthe preformed parison 110 with a mandrel producing the inner diameterand the perform mold producing the outer shape. These preformed parisonscan then be packaged, after cooling. In the stretch blow moldingprocess, the preformed parison is heated, typically using infraredheaters, above its glass transition temperature. Then, in a blow mold,the parison is blown into the finished shaped noncellular article usinghigh pressure air, while being stretched with a plunger as part of theprocess 118. The stretching of some thermoplastic materials, such aspolyethylene terephthalate, results in strain hardening of the material.

A process is disclosed for producing a cellular blow molded article froma solid thermoplastic material by a modification of the blow molding andstretch blow molding processes. The process disclosed herein isapplicable to both blow molding and stretch blow molding. The disclosedprocess includes the steps of:

(a) treating a solid parison made from a thermoplastic material with asaturating gas at an elevated pressure for a period of time to provide agas-saturated parison;

(b) heating the gas-saturated parison to prepare a cellular parison;

(c) placing a cellular parison in a mold; and

(d) blowing a molding gas into the cellular parison to expand thecellular parison into the shape of the mold to provide a shaped cellulararticle.

The disclosed process modifies the traditional blow molding and stretchblow molding processes by treating a solid (nonmolten) and noncellularparison with a saturating gas before the parison is heated and blowmolded. For example, the solid and noncellular parison can be made inthe traditional manner, but then allowed to solidify and treated withthe saturating gas. The conventional injection blow molding processproduces a molten not solid injection-molded parison, which is then blowmolded. The disclosed process results in a blow molded cellular article.

Referring to FIG. 2, a solid and noncellular preformed parison 200 isobtained made from a thermoplastic material using processes well-know inthe plastics industry. The preformed parison 200 can be made accordingto the description described above in connection with FIG. 1, blocks100, 105 and 110. The thermoplastic material can be any singlethermoplastic polymer or a mixture of thermoplastic polymers including,but not limited to, polycarbonate, polypropylene, polyethylene,polyethylene terephthalate, polyvinyl chloride, poly(lactic acid),acrylonitrile butadiene styrene, and polystyrene.

From block 200, the process enters block 210. In block 210, the solidand noncellular parison is treated at an elevated pressure with asaturating gas. The treatment of the solid parison at an elevatedpressure causes the thermoplastic material to absorb the saturating gas,leading to a gas-saturated parison. The treatment can proceed tocomplete saturation followed by a step for desorption, or alternatively,the treatment can proceed to partial saturation followed by a step fordesorption. Desorption can be incidental to the process or anintentional step in the process. If the desorption is incidental, then,the desorption period is the time from removal of the parison from theheating device to the blow mold. Desorption results in lower gasconcentrations which can be used to create solid surfaces where the gasconcentration is insufficient to create a cellular structure. Thesaturating gas may be an inert gas including, but not limited to, carbondioxide, nitrogen, or any combination thereof. In one embodiment, thehigh pressure saturating gas includes carbon dioxide. In one embodiment,the high pressure saturating gas includes nitrogen. The elevatedpressure may be from about 3 MPa to about 7.5 MPa. In one embodiment,the elevated pressure is about 4 MPa. In another embodiment, theelevated pressure is about 5 MPa.

The treatment of the solid parison in block 210 may be carried out in apressure vessel filled with a saturating gas. When the pressure vesselis sealed, the material is exposed to a high pressure saturating gas.The high pressure gas will then start to diffuse into the thermoplasticpolymer over time, filing the thermoplastic polymer's freeintermolecular volume. The gas will continue to saturate thethermoplastic polymer until equilibrium is reached. Therefore, dependingon the length of time the parison is treated with the saturating gas,the parison may be fully saturated with the saturating gas.Alternatively, the parison may be partially saturated with thesaturating gas. Depending on the size and thickness of the walls of theparison and the pressure of the saturating gas, the duration oftreatment of the parison with high pressure saturating gas may vary fromabout 2 hours to about 60 days. In one embodiment, the treatment lastsfrom about 15 days to about 25 days. In another embodiment, thetreatment lasts for about 21 days. The amount of time for completesaturation can be determined beforehand. For example, a test using thepolymer parison to be blow molded can be conducted at varioustemperature and pressure conditions and sampled at various timeintervals. The sample can be pulled from the pressure vessel andmeasured for weight. When the weight of the sample ceases to increaseover time, the sample has reached complete saturation for the giventemperature and pressure. The time can be noted, and various tables forachieving complete saturation can be created for any given combinationof temperature and pressure conditions.

During treating in block 210, a plurality of solid parisons may betreated simultaneously at an elevated pressure to provide a plurality ofgas-treated parisons. The parisons may be nested one inside another orpartially nested one inside another. Each parison may have a shape thatis smaller at the bottom than at the top to allow nesting of one parisoninside another. Furthermore, the parisons can be shaped to nest insideeach other in such a way that the saturating gas can come into contactwith the entire surface of each parison, eliminating the need for aporous membrane between parisons

From block 210, the method may alternatively proceed to block 215,desorption. Because the gas-treated parison is moved to an environmentof lower pressure, the thermoplastic material of the gas-treated parisonbecomes thermodynamically unstable, which means that the thermoplasticmaterial is no longer at equilibrium with the surrounding environmentand that the thermoplastic material becomes supersaturated with thesaturating gas. The gas-treated parison will start to desorb gas fromits surface into the surrounding environment. In one embodiment, aftertreating with the saturating gas and before heating, the parison isallowed to partially desorb gas. The desorption of some of the gas, insome circumstances, helps to avoid creation of the cellular structure incertain areas of the parison, such as at the surface. Desorption canoccur when the high-pressure saturating gas is vented from the pressurevessel or when the gas-treated parison is removed into ambientatmosphere pressure.

From block 210 and omitting step 215, or alternatively from block 215,the method proceeds to block 220, heating. In block 220, the gas-treatedsolid parison is heated to produce a cellular parison. The parison orparisons may be heated with any heating methods and apparatusesincluding, but not limited to, infrared heating and air impingementoven. Heating of the gas-treated parison in block 220 may be carried outat a temperature below the melting temperature of the thermoplasticmaterial. The heating produces a cellular and solid parison. Thecellular parison may have uniform wall thickness with nucleated bubblesformed within the parison wall. The heating temperature will depend onthe type of thermoplastic materials. For example, the heatingtemperature may be from about 50° C. to about 175° C. for a parison madefrom polyethylene terephthalate; the heating temperature may be fromabout 50° C. to about 150° C. for a parison made from polyvinylchloride; the heating temperature may be from about 40° C. to about 125°C. for a parison made from poly(lactic acid); the heating temperaturemay be from about 50° C. to about 125° C. for a parison made fromacrylonitrile butadiene styrene; the heating temperature may be fromabout 50° C. to about 150° C. for a parison made from polystyrene, theheating temperature may be from about 50° C. to about 150° C. for aparison made from polycarbonate, the heating temperature may be fromabout 100° C. to about 200° C. for a parison made from polypropylene,and the heating temperature may be from about 75° C. to about 150° C.for a parison made from polyethylene. In one embodiment, the heatingtemperature is about 110° C. for a parison made from polyethyleneterephthalate.

From block 220, the method proceeds to block 225. In block 225, thecellular parison is blow molded. Blow molding is a step in which thecellular parison is placed in a mold and further heated to a temperatureabove the melting or softening point of the parison and then the parisonis expanded with a molding gas into the shape of the mold to provide thefinished thermoplastic cellular article. The blow molding step 225 mayalternatively include mechanical stretching 218 of the parison, such aswith a plunger discussed above. A person skilled in the art wouldreadily appreciate that any inert gas could be useful as a molding gas.In one embodiment, the molding gas is compressed air. Additionally,other gases useful for expanding the cellular parison include, but arenot limited to, nitrogen, argon, xenon, krypton, helium, carbon dioxide,or any combination thereof. The parison or parisons may be heated inblock 225 by applying heat to the mold.

The parison heating that takes place during the blow molding step 225may cause further formation of nucleated bubbles, i.e., foaming, in thethermoplastic material of the parison. The foaming continues during theblow molding process, resulting in a cellular thermoplastic article 240as the finished product after a cooling period, block 230.

Referring to the diagrammatical illustration of FIG. 6, in oneembodiment, the cellular thermoplastic article comprises cells having asize of from about 5 μm to about 200 μm. In another embodiment, thecellular thermoplastic article comprises cells having a size of fromabout 5 μm to about 150 μm. In another embodiment, the cellularthermoplastic article comprises cells having a size of from about 50 μmto about 100 μm. In a further embodiment, the cellular thermoplasticarticle comprises cells having a size of from about 50 μm to about 150μm.

The process described in relation to FIG. 2 may further comprisecooling, block 230 after blow molding, block 220, the cellular articleto a temperature below the melting or softening point of the shapedcellular article.

Referring to FIG. 3, a diagrammatical illustration of blow moldingequipment is illustrated. In this FIGURE, the solid parison 300 madefrom a thermoplastic material is placed in the sealed pressure vessel310 with a saturating gas CO₂ at an elevated pressure for a period oftime. The solid parison 300 is then removed from the pressure vessel 310and placed into a heating device 320. The heating device can be an oilbath, however, other heating apparatuses may be used. The parison foamswhen heated to provide a cellular and solid parison 330 with uniformwall thickness. In the embodiment shown, the solid parison 300 has aneck portion 305 and a body portion 307. The neck portion may comprisescrew threads, such as would be used to produce a bottle or jar. Duringthe heating step, only the body portion 307 is heated resulting in acellular parison 330 having a cellular body portion 307 and a solid neckportion 305. When heating the gas-treated parison, only the body portionof the parison is heated, resulting in a cellular parison having acellular body portion and a solid neck.

The cellular parison 330 is then clamped into a metal mold 340. Themetal mold 340 may be further heated to a temperature above thesoftening point of the cellular parison 330. A collar 355 with aninjection port 365 is attached to the neck portion of the parison 330. Ahollow plunger 360 resides in and slides within the collar, such thatmolding gas blown through injection port 365 passes into the hollowparison, either directly into the parison or though the plunger 360 andis distributed within the interior of the parison 330 either directlythrough the mouth of the parison or through the plunger 360. The moldinggas 350 expands the cellular parison 330 into the shape of the mold 370.The molding gas can be compressed air. When heated through the mold 340,further nucleated bubbles can form in the thermoplastic material of theparison during the blow molding process to provide a shaped cellulararticle 380. After blow molding, the cellular article 380 is cooled to atemperature below the softening point of the shaped cellular article andthen ejected from the mold 330.

Referring to FIG. 4, a diagrammatical illustration of a pressure vessel400 holding a plurality of parisons is shown. A plurality of cup-shapedparisons 402 and 404 are treated in the gas-filled pressure vessel 400.The parisons are nested inside each other with parison 404 nested insideparison 402. There are four tabs 406 on the outside wall of eachparison. The tabs serve to keep a distance between the outer surface ofthe parison 404 and the inner surface of the parison 402 while allowingparison 404 to nest inside parison 402 stably. Therefore, the saturatinggas can come into contact with the entire surface of both parison 404and parison 402.

Compared to the traditional blow molding processes, the disclosedprocess provides several advantages. First, by forming a cellularthermoplastic article, the process provides material savings from usingcellular instead of solid plastic. Second, the cellular thermoplasticarticles, such as bottles and jars, possess excellent insulatingproperties, which is useful in keeping the temperature constant of thecontents in the bottles or jars. For example, the cellular thermoplasticbottles and jars produced by the disclosed process have the superiorproperty of keeping hot beverages hot and cold beverages cold. In oneembodiment, the disclosed process could be used to produce cellularthermoplastic inserts for coffee bottles or jars. Third, the disclosedprocess may utilize conventional injection or stretch blow moldingequipment with minor modifications, therefore providing a simple andefficient process for producing a cellular thermoplastic article.Fourth, the saturating gas trapped in the thermoplastic material servesas a plasticizer, allowing cooler temperatures to be used during theblow molding process, and aiding in thermoplastic stretching overall.Finally, upon the completion of the blow molding process, the saturatinggas, such as carbon dioxide, trapped inside the cells of the cellularstructure of the finished thermoplastic article, aids in quick coolingof the blow molded article.

EXAMPLES Example 1

A polyethylene terephthalate (PET) parison was saturated for 21 days ina pressure vessel with CO₂ gas at 4 MPa. The parison was removed fromthe pressure vessel, allowed to desorb gas at ambient temperature andpressure for 5 minutes and heated in a heated oil bath at 110° C. for120 seconds. The parison foamed in the oil bath to uniform wallthickness to provide a cellular parison. Large bubbles were visible. Theparison retained its shape with no visible distortion from the verticalaxis during the foaming process. The cellular parison was then blowmolded into a hollow article using 60 psi air flow from a compressed airline. The compressed air stretched the foamed parison fully into thehollow article, creating a well formed solid-state cellular polyethyleneterephthalate bottle.

Example 2

A polyethylene terephthalate (PET) parison was saturated for 21 days ina pressure vessel with CO₂ gas at 4 MPa. The parison was removed fromthe pressure vessel, allowed to desorb gas at ambient temperature andpressure for 5 minutes and heated in a heated oil bath at 110° C. for 90seconds. The parison foamed in the oil bath to uniform wall thickness toprovide a cellular parison. Large bubbles were visible. The parisonretained its shape with no visible distortion from the vertical axisduring the foaming process. The cellular parison was then blow moldedinto a hollow article using 30 psi air flow from a compressed air line.The compressed air did not fully stretch the cellular parison into thehollow article.

In example 2 the compressed air pressure was not sufficient to overcomethe strength of the cellular parison's strength at that temperature andhence could not be stretched into the shape of the blow mold.

While illustrative embodiments have been illustrated and described, itwill be appreciated that various changes can be made therein withoutdeparting from the spirit and scope of the invention.

1. A process for producing a cellular article from a thermoplasticmaterial, comprising: (a) treating a solid parison made from athermoplastic material with a saturating gas at an elevated pressure fora period of time to provide a gas-saturated parison; (b) heating thegas-saturated parison to prepare a cellular parison; (c) placing thecellular parison in a mold; and (d) blowing a molding gas into thecellular parison to expand the cellular parison into the shape of themold to provide a shaped cellular article.
 2. The process of claim 1,wherein the saturating gas comprises carbon dioxide, nitrogen, or anycombination thereof.
 3. The process of claim 1, wherein thethermoplastic material comprises polycarbonate, polypropylene,polyethylene, polyethylene terephthalate, polyvinyl chloride, polylacticacid), polycarbonate. polypropylene, acrylonitrile butadiene styrene, orpolystyrene.
 4. The process of claim 1, wherein the elevated pressure isabout 3 MPa to about 7.5 MPa.
 5. The process of claim 1, wherein step(b) comprises heating the gas saturated parison to a temperature ofabout 110° C.
 6. The process of claim 1, wherein the molding gas isselected from the group consisting of air, nitrogen, argon, xenon,krypton, helium, and carbon dioxide.
 7. The process of claim 1, whereinstep (a) comprises saturating a plurality of solid parisons with thesaturating gas simultaneously to provide a plurality of gas-saturatedparisons.
 8. The process of claim 7, wherein the plurality of solidparisons are nested one inside another.
 9. The process of claim 7,wherein the plurality of solid parisons are partially nested one insideanother.
 10. The process of claim 9, wherein each parison has a shapethat is smaller at the bottom than at the top to allow nesting of oneparison inside another.
 11. The process of claim 7, wherein theplurality of solid parisons are nested one inside another whileproviding for the saturating gas to contact the entire surface of allparisons.
 12. The process of claim 1, wherein the solid parisoncomprises a body portion and a neck portion.
 13. The process of claim12, wherein the neck portion further comprises screw threads.
 14. Theprocess of claim 12, wherein step (b) comprises heating the body portionof the gas-saturated parison to provide the cellular parison having acellular body portion and a solid neck portion.
 15. The process of claim1, further comprising allowing the saturating gas to partially desorbafter step (a) and before step (b).
 16. The process of claim 1, whereinthe parison is fully saturated with the saturating gas during treating.17. The process of claim 1, wherein the parison is partially saturatedwith the saturating gas during treating.
 18. The process of claim 1,wherein the cellular article comprises cells having a size of from about5 μm to about 200 μm.
 19. The process of claim 1, further comprisingheating the cellular parison while in the mold.
 20. The process of claim1, further comprising heating the cellular parison during step (d) tofurther create cells.
 21. The process of claim 1, further comprising thestep of injecting a molten thermoplastic material into a parison moldthen cooling to provide a solid parison before step (a).
 22. The processof claim 1, wherein step (d) further comprises stretching the cellularparison with a plunger while blowing the molding gas into the cellularparison.
 23. The process of claim 1, further comprising cooling theshaped cellular article to a temperature below the melting or softeningpoint of the shaped cellular article after step (d).